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The protonation constants of the betanin (pKa1, pKa2, and pKa3) were determined in mixed solvent of water and methanol containing 0, 10, 20, 30, 40, 50, 60, 70, and 80 % (v/v) methanol, using a combination of the spectrophotometric and potentiometric methods at T = 25 oC and constant ionic strength (0.1 mol.dm-3 NaClO4). The obtained protonation constants were analyzed using Kamlet, Abboud, and Taft parameters. KAT parameters are α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and π* (dipolarity/polarizability). In this study, a good linear relationship was obtained between protonation constants (on the logarithmic scale) and dielectric constant (ɛ) of the water-methanol mixed solvents. It was found that the dualparameter correlation between log10K,s and π*, β give us the best result in various volume fractions of methanol for water-methanol mixed solvent. Finally, the results are discussed in terms of the effect of the solvent on the protonation constants.

Solvent Effects on Protonation Constants of Betanin in Different Aqueous Solutions of Methanol at T = 25 ºC